A typical form of freight loading system, which is used for example in an air freight traffic context, comprises a conveyor surface over which are distributed an array of drive roller units, each of which has a motor for driving same, for displacing articles to be loaded or unloaded, such as freight containers. Thus, such a system may be used for loading freight containers into a freight compartment of an aircraft and conveying the freight containers within the freight compartment to respective predetermined locations intended therefor. Such as freight loading system will often include a control assembly for controlling operation of the drive roller units.
In general the freight compartment of an aircraft is elongate in the longitudinal direction of the aircraft and at one longitudinal side at least has an entry door through which the freight container can be introduced. The floor of the freight compartment constitutes the conveyor surface, being provided with roller-type and/or ball-type conveyor tracks or the like, and the motor-driven drive roller units are distributed thereover, to move the container when it is introduced through the loading door into the freight compartment and then move it in the longitudinal direction thereof to its intended destination.
Usually, drive roller units of that kind are arranged in the door region of the freight compartment to provide for transverse conveying, that is to say transversely with respect to the longitudinal axis of the freight compartment or the aircraft, and they are then followed in the longitudinal direction by mutually parallel rows of drive roller units for conveying articles such as freight containers in the longitudinal direction of the aircraft. Now, so that the freight container when introduced through the door into the freight compartment by the transversely conveying drive roller units can then be driven in the longitudinal direction of the freight compartment, it is necessary for the bottom surface of the freight container to cover not only the transversely conveying drive roller units in the door region, but also the longitudinally conveying drive roller units disposed in the region of the door, which adjoin the transversely conveying drive roller units in the longitudinal direction. If that were not the case, the container, after having been introduced through the door in the transverse direction, could not be further conveyed within the freight compartment in the longitudinal direction.
In a practical context a freight loading system of that kind gives rise to a number of problems.
Usually, such freight loading systems are equipped with drive roller units which in functional terms approximately correspond to those disclosed in German patent application No. P 37 24 126.5 (corresponding to U.S. Pat. No. 4,792,037) to which reference may therefore be directed.
If with that arrangement the freight loading system is activated for loading a freight container, all the drive rollers of the drive roller units firstly rise out of their lowered rest position into a raised operative position and begin to rotate. The freight container is therefore conveyed transversely to the longitudinal axis of the aircraft by the drive roller units which operate in the transverse direction, in the region of the door, until the freight container reaches the opposite side wall of the freight compartment. In order now to avoid overloading of the transversely conveying drive roller units by virtue of the freight container running against the side wall of the freight compartment, either the prior arrangement has limit switches which switch off the transversely conveying drive roller units when the freight container encounters the side wall of the freight compartment which is opposite the door, or there is an external control switch which must be actuated by an operator as soon as the freight container begins to encounter the side wall of the freight compartment. It is unfortunately found in practice that under the rough operating conditions of a freight loading system the limit switches frequently suffer damage and fail so that excessive loadings which occur when the container encounters the freight compartment side wall cause overloading of and progressive damage to the drive roller units involved. A switch which can be manually actuated by an operator provides only a limited remedy in this respect for it may happen that the operator can easily forget to actuate the manual switch.
A further problem which is often particularly serious can arise if, as already mentioned above, when the container is being loaded into the freight compartment of the aircraft through the door, the bottom surface of the container comes into contact not only with transversely conveying drive roller units but also longitudinally conveying drive roller units which are necessarily arranged adjacent thereto. It is therefore absolutely necessary to ensure that, when the container is loaded in the transverse direction, the adjacent longitudinally conveying drive roller units are moved into the lowered position. In the prior arrangement that is achieved by suitable switches which are disposed in the path of movement of the container or by manual actuation on the part of an operator. In that respect the same considerations as those indicated above also apply. The longitudinally conveying drive roller units which are disposed in the door region may frequently suffer damage as a result.
Similar considerations as those set forth above in relation to the transversely conveying drive roller units also apply in relation to the longitudinally conveying drive roller units which are at the front and rear ends of the freight compartment as, when a freight container encounters an end wall of the freight compartment, it is necessary to ensure that the longitudinally conveying drive roller units which are still in operative engagment with the underneath surface of the freight container are switched off in order to prevent overloading and overheating.
In conventional freight loading systems, all drive roller units, or at least in each case the transversely conveying units or all the longitudinally conveying units, are simultaneously switched into the operative conveying condition although there is only ever a small number of drive roller units that contribute to conveying of the freight container, namely those which are disposed beneath the underneath surface of the container at any given time. That gives rise to the problem that the other drive roller units which are running in an idle condition not only consume power unnecessarily but in addition for most of the time are also uselessly in the idle drive condition so that their service life is reduced.